1. Field of Invention
The invention relates to the bleaching of high-yield lignin-containing pulps to be used for papermaking. It is concerned with the use of di-oxygen (O.sub.2), an alkaline agent, and borohydride (also known as tetrahydroborate, BH.sub.4 -) to produce high brightness and low yellowness pulps.
2. Description of the Prior Art
Di-oxygen in the presence of an alkaline agent, for example, alkali (O.sub.2 /OH-) is used commercially to bleach wood pulp or cotton pulp but not high-yield pulps. O.sub.2 /OH- bleaches chemical pulps by degrading and dissolving the residual lignin (see for example U.S. Pat. No. 1,163,438 which describes the bleaching of cotton by alkali and oxygen and U.S. Pat. No. 4,248,662 which uses oxygen, alkali and borate salts to bleach wood pulps by delignifying them).
In high-yield pulps that retain the lignin, bleaching by lignin removal is impractical and lignin-containing pulps darken rather than bleach under the influence of di-oxygen and alkali, D. H. Andrews et al, "The Bleaching of Pulp", Ed. Tappi Press (1979) p. 215; and T. S. Khristov and T. Semov "Oxygen-alkali treatment of High-temperature Thermomechanical Pulp", Tseluloza Khartiya 13 (5) 18-21 (1982).
By contrast, the bleaching of high-yield pulps with added alkaline hydrogen peroxide [H.sub.2 O.sub.2 /OH-] is effective and is widely used by the pulp and paper industry. The efficacy of mechanical pulp bleaching using added alkaline peroxide is marginally improved if di-oxygen is present. U.S. Pat. No. 4,756,798 describes that there is an increase of 1-2 brightness points, but also that it is not known why this occurs.
Inserting a di-oxygen/alkali ("O") stage before an alkaline peroxide ("P") bleaching stage gives a lower brightness than alkaline peroxide alone, Michell et al, Bleaching and Yellowing of Eucalypt Chemimechanical Pulps. Diffuse Reflectance Spectra of Oxygen Bleached Pulps, Appita 44 333-336 (1991).
Single stage bleaching with 4% added alkaline peroxide increases the brightness of mechanical pulps made from Canadian spruce fibres by about 20 points and ISO brightnesses up to about 80% can be obtained by repetitive treatments. The main disadvantages of adding hydrogen peroxide are that it is unstable (both during bleaching and in storage) and must be used in conjunction with a number of additives, for example, magnesium sulphate; sodium silicate; and chelating or sequestering agents such as DTPA or EDTA. The need for these additives significantly increases the cost of using the peroxide. Also the addition of silicates may lead to unwanted deposits when mill water is recycled and the discharge of DTPA or EDTA in effluents is environmentally undesirable.
Di-oxygen is stable and of lower cost than peroxide. It is readily available and is generally beneficial to the natural environment. A di-oxygen-based bleaching system could avoid the use of some or all DTPA, EDTA, magnesium sulphate or silicates. It would be a step forward if oxygen could be used in place of peroxide.
There are a number of reports and patents on the use of sodium borohydride (NaBH.sub.4), alone or in sequence with a second stage using added hydrogen peroxide, to bleach high yield pulps:
(i) Lords and Soteland, "High Brightness Bleaching of Mechanical Pulp", Norsk Skogindustri 26 (10) 255-258 (1972), achieved an optimum brightness increase of 12.5 pts. when spruce groundwood was treated with 1.2% NaBH.sub.4 alone. They also reported that two stage bleaching with added peroxide in the second stage gave a maximum total increase of 20 pts. PA0 (ii) Smedberg, U.S. Pat. No. 3,100,732 describes the combined use of borohydride and added alkaline peroxide, reporting higher groundwood brightnesses of 1.2 (0.1% NaBH.sub.4) and 4 (1% NaBH.sub.4) relative to the brightness obtained when added alkaline peroxide was used alone. PA0 (iii) Kruger et al., U.S. Pat. No. 3,981,765, used borohydride to impregnate wood chips prior to heating and mechanically defiberizing. PA0 (iv) Polcin and Rapson, "Effects of Bleaching Agents on the Absorption Spectra of Lignin in Groundwood pulps; Part 1. Reductive Bleaching" Pulp and Paper Magazine of Canada 72 (3) T103-T125, (1971), obtained a brightness increase of 5.8 points when eastern spruce groundwood was treated with 0.45% NaBH.sub.4.
There is a report of the use of di-oxygen as part of a bleaching sequence, in a separate stage from borohydride. Michell et al referred to hereinbefore bleached eucalypt chemimechanical pulps in three stages with a NaBH.sub.4 -di-oxygen-peroxide ("B--O--P") sequence. Addition of a "B" stage prior to a "P" stage improved brightness by 3.5 pts. but addition of a di-oxygen "O" stage between the "B" stage and the "P " stage led to a 2 pts. brightness loss.
Sodium borohydride is frequently used to generate hydrosulphite for mechanical pulp bleaching by reaction with sulphur dioxide or sodium sulphite. U.S. Pat, No. 3,284,283 describes the in situ use of borohydride and sodium bisulphite to generate hydrosulphite on the pulp. This is a completely different use of borohydride from that described herein, which involves reaction in the presence of oxygen and the absence of sulphur compounds.